1. Field of the Invention
This invention relates to a deviation amount detecting device which computes an amount of deviation for each of multiple toner images of different colors in a color image forming device wherein a color image is formed by superimposing the toner images of different colors.
2. Description of the Related Art
In a tandem type color image forming device, a color image is formed on a recording sheet or an intermediate transfer belt by using four image formation units of different colors which are arranged to superimpose the toner images on one another on the recording sheet or the intermediate transfer belt.
In the image forming device of this type, if the position where the toner images of the respective colors are superimposed slightly deviates from a desired position, it is difficult to stably obtain a color image with good quality. To avoid this problem, deviation compensation patterns of the respective colors formed on a transporting member are detected, and the deviation compensation is performed so that the toner images of the respective colors are superimposed at the same position. Specifically, by this deviation compensation, each of the detection results of color patterns (cyan, magenta and yellow) is compared with the detection result of a reference color pattern (black), and an amount of deviation of each color pattern to the reference color pattern is computed.
However, even if the computation of the amount of deviation and the deviation compensation are performed, a deviation will take place again according to various factors with the passage of time. Especially, if the reflection characteristics of the reflection mirror of the image forming device change due to a temperature rise of the exposure unit of the image forming device, a deviation may easily take place.
Conventionally, in order to correct the deviation which takes place due to the temperature rise of the exposure unit, it is necessary to frequently perform a deviation compensation process using the deviation compensation patterns. Refer to Japanese Laid-Open Patent Application No. 2005-103927 and Japanese Laid-Open Patent Application No. 2006-259444.
However, the deviation compensation process using the conventional deviation compensation patterns needs to form many color patterns on a transporting belt, needs to read these color patterns by the sensors, and needs to perform the computation to compute the amounts of deviation based on the results of reading of the color patterns. Thus, the deviation compensation process using the conventional deviation compensation patterns requires a series of several tasks, including the formation of color patterns on the transporting belt, the reading of the color patterns by the sensors and the computation based on the pattern reading results, and much time is needed to complete the deviation compensation process.
The amount of deviation for one of the different colors produced due to a temperature rise in the exposure unit of the image forming device is different in size from that for another of the different colors. However, the deviation compensation process using the conventional deviation compensation patterns performs the deviation compensation uniformly for all the colors. There is a problem in that the deviation compensation process using the conventional deviation compensation patterns includes an unnecessary compensation process, which is not efficient.